The present invention relates to a spark plug for internal combustion engines.
In recent years, there is an increasing demand for a highly durable spark plug for internal combustion engines as the engines become higher in efficiency. Under these circumstances, the durability of the ground electrode is one of the most important problems to be solved. There are two problems with respect to the durability required for the ground electrode. One problem is concerned with the resistance to electrode consumption due to spark discharge. Another problem is concerned with the strength. Namely, there is a problem that transmission of engine vibrations to the ground electrode may possibly cause a crack at the welded portion between the ground electrode and the metallic shell and the progress of the crack may cause the ground electrode to be broken.
With reference to FIGS. 10A-10C and 11A-11C, the structure of the ground electrode will be described.
As shown in FIG. 1A, a welded portion 16c of a ground electrode 16 connected to a front end face 37 of a metallic shell 30 is generally rectangular in cross section and has the maximum width L1 and the maximum thickness t1. As shown in FIG. 10B, assuming that M denotes the nominal diameter of an externally threaded portion 31 of the metallic shell 30, A denotes the outer diameter of the front end face 37 and B denotes the inner diameter of the front end face 37, a spark plug of 12.00 mm in the nominal diameter M of the externally threaded portion 31 is, for example, 10.10 mm in the outer diameter A of the front end face 37. In this instance, if the inner diameter B is 7.20 mm, the welded portion 16c can be sized to be 1.30 mm in the maximum thickness t1 and 2.70 mm in the maximum width L1. In the meantime, as shown in FIG. 10B, a clearance 24a is provided between the inner circumferential surface of the metallic shell 30 and the outer circumferential surface of an insulator front end portion 24.
Herein, as a means for improving the resistance to consumption of the ground electrode 16, it is considered to expand the maximum width L1 of the ground electrode 16 and thereby make larger the area of a side surface portion 16g facing the center electrode 12. Further, as a means for making the strength of the ground electrode 16 higher, it is considered to make the maximum thickness t1 thicker and thereby make the rigidity of the ground electrode 16 higher.
However, if the maximum width L1 of the ground electrode 16 is made wider and the maximum thickness t1 is made thicker, the side surface 16a including the side surface portion 16g facing the center electrode 12 protrudes from the inner circumferential periphery 37b of the front end face 37, and the opposite end corner portions 16e of the side surface 16b opposite to the side surface 16a protrude from the outer circumferential periphery 37a of the front end face 37 as shown in FIG. 11B. To eliminate such protrusion, it is necessary to make the metallic shell 30 thicker and thereby make the area of the front end face 37 larger. In this connection, the width of the front end face 37 can be made larger by making the outer diameter A of the end face 37 large or the inner diameter B smaller. However, increase of the outer diameter A inevitably results in increase of the nominal diameter M of the externally threaded portion 31 of the metallic shell 30. This disables attachment of the spark plug to the cylinder head, so the outer diameter A cannot be made larger.
If the inner diameter B is reduced, as shown in FIG. 11C, from the position indicated by 37b to the position indicated by 37c, the clearance 24a between the inner circumferential surface of the metallic shell 37 and the insulator front end portion 24 will inevitably reduce. By this, sideway jumping of spark due to adherence of carbon onto the surface of the insulator front end portion 24 is liable to be caused, thus causing a problem that the ignition ability is lowered.
It is accordingly an object of the present invention to provide a spark plug for internal combustion engines which can improve the resistance to electrode consumption and the strength of the ground electrode without changing the inner and outer diameters of the end face of the metallic shell.
To accomplish the above object, the present invention provides a spark plug for an internal combustion engine comprising a metallic shell having an externally threaded portion, an insulator disposed in the metallic shell and having an axial bore, a center electrode disposed in the axial bore of the insulator, and a ground electrode connected to a front end face of the metallic shell and having an end opposite to an front end face of the center electrode, wherein a cross section of the ground electrode is so shaped as to provide a side surface at one of opposite sides which faces an outer circumferential periphery of the front end face of the metallic shell, with a narrower central side surface section than that of a side surface at the other of the opposite sides, the central side surface section at one of the opposite sides being parallely opposite to the central side surface section at the other of the opposite sides.